I had the use of a 12" 445 Super Mag barrel with an OTT Sharkgill break for a summer. I would say it reduced recoil by 50%. I shot the barrel a couple of times without the break, lets just say it was handfull. I have also shot a 15" 500 S&W, 50 Alaskan, and a 458 Win Mag barrel with the Sharkgill break. It really tamed the beast.
I was shooting the Rem 240gr SJHP @ 2,000 fps from the 12" barrel. I happened to get a charlie with it.